Congenital defects, disease, or injury can render a person's blood vessels incapable of serving as an appropriate conduit for blood. Autogenous blood vessels may be relocated from their original site in the person's body and grafted to a new site as a replacement for the diseased or traumatized native vessel. Synthetic or non-autogenous tissue origin vascular grafts may also be implanted in a person to replace diseased or traumatized native vasculature.
Infection, aneurysm, thrombosis, hyperplastic tissue response, stenosis at the anastomoses, are all problems which occur with any known vascular graft whether it be of autogenous or non-autogenous origin. Long term patency of vascular grafts is dependent upon many factors including the skill of the implanting surgeon, patient health status, quality of the vascular graft and other factors. The influence of any particular factor on successful long term function has not been quantified. However, it is commonly held that low velocity of blood flow and low flow rate are major factors which reduce patency. Conversely then, higher flow rate and higher velocity of flow are major factors which increase patency longevity.
The autogenous saphenous vein is used successfully as a vascular conduit for coronary artery revascularization. Although the search for a suitable prosthetic graft for aortocoronary bypass continues, nothing better than the autogenous saphenous vein is available. Surgeons have been reluctant to use synthetic grafts in aortocoronary bypass because of few proved instances of long-term patency.
Although saphenous veins are used in aortocoronary bypass procedures, there are certain disadvantages: (1) unavailability, (2) small size, (3) non-uniform caliber, (4) varicosities, (5) large diameter, (6) sclerosis, (7) obstruction due to intimal hyperplasia, (8) aneurysm formation, (9) considerable time required for harvesting, (10) leg discomfort and swelling, and (11) possible leg infection.
A significant number of patients requiring aortocoronary bypass do not have suitable veins, or the veins have been used for previous aortocoronary bypass or for peripheral vascular bypass procedures. On occasion, the need for a graft may have been unforeseen prior to surgery, and the legs not prepared for harvesting of the vein. The cephalic vein from the arm has been used when the saphenous vein is not available. However, it is usually thin-walled and often of poor caliber. Furthermore, the cosmetic effect of harvesting the cephalic vein is unacceptable for some patients.
The internal mammary artery is widely accepted as suitable for myocardial revascularization, in that it has an excellent patency ratio, but is useful only for the left anterior descending and diagonal coronary arteries. Experience with free grafts of the internal mammary and radial arteries has been disappointing, since long-term patency has been poor.
The importance of the velocity of blood flow in autogenous vein grafts has been emphasized. There is evidence of an inverse relationship between the velocity of blood flow in venous grafts and the amount of intimal proliferation observed. Autopsy studies indicate that occlusion of aortocoronary saphenous vein grafts more than one month after operation is most commonly caused by fibrous intimal proliferation. Although the cause of this lesion has not been definitely established, studies would suggest that it is probably related to a low velocity of flow through the graft. This suggests that every effort should be made to achieve a high velocity of flow in coronary artery bypass grafts.
Synthetic vascular implants are disclosed by Liebig in U.S. Pat. Nos. 3,096,560; 3,805,301 and 3,945,052. These grafts are elongated knit fabric tubes made of yarn, such as polyester fiber. Dardik in U.S. Pat. No. 3,894,530 discloses the use of an umbilical cord for a vascular graft. Holman et al in U.S. Pat. No. 4,240,794 disclose a method of preparing human and other animal umbilical cords for use as a vascular replacement. The fabric tubes and umbilical cords have been used to replace the saphenous vein implant. The ends of the tubes and cords are anastomosed to ends of arteries to by-pass diseased areas of the arteries. They replace the diseased portions of the arteries.